The present invention relates generally to magnetic resonance (MR) angiography, which is the MR imaging of an artery or like vessel carrying blood and other fluid. More particularly, the invention pertains to a method of acquiring MR data at each of a number of scan locations or stations, which are spaced along the peripheral vasculature of a patient. Data is acquired after an initial test bolus of contrast agent is injected into the patient and is timed as it travels along the vessel or other conduit, from station-to-station. After the bolus travel time is known, an exam bolus is injected and MR data is acquired at each scan station while the bolus is located there.
It is a well known practice in MR angiography to insert a volume of contrast agent, such as gadolinium chelate, into blood flowing along a vessel. The volume or mass of contrast agent is referred to as a bolus, and has the effect of shortening the T1 time of the blood. Thus, an MR image of the blood, acquired by a fast gradient echo or similar technique, will show up very well with respect to adjacent stationary tissue of the vessel structure. It is also well known, when imaging a blood vessel of comparatively great length, to acquire MR data from a patient at a given number of stations or scan locations, which are located at intervals along the vessel. To acquire MR data at a particular station, the patient is selectively positioned with respect to an MR scanner, typically by moving a patient table. Data is then acquired from a series of slices taken through a region or section of the patient in the particular scan location or station. Thereafter, the patient is shifted, relative to the scanner, so that data may be acquired from another section of the patient, in another scan station. MR angiography employing this procedure in conjunction with an injection of a contrast bolus may be referred to as bolus chasing peripheral MR angiography.
At present, when a contrast agent is used in connection with a peripheral MR angiography exam, the first scan station is selected to be the section of the patient, along a vessel of interest, at which the bolus arrives first. When the scan at the first station is completed, the acquisition normally moves to the next scan station. However, the most appropriate time to move to the next station is not precisely known. For example, in the case of slow blood flow, the distal vasculature at the next scan station may not have had adequate time to fill with contrast material. On the other hand, if flow rate is greater than anticipated, the contrast agent may tend to move into stationary tissue adjacent to the next scan station, before data acquisition commences. In either case, contrast between moving fluid and stationary vessel tissue may be significantly reduced at the next scan station. Moreover, undesirable effects, resulting either from flow rate which is too slow or too great, may tend to become progressively worse as imaging proceeds to subsequent scan stations and as the total number of scan stations increases.
Further, since the maximum safe dose of the contrast material cannot be exceeded, the number of scan sections or stations that can be imaged is limited and if an image is acquired either too early or too late, with respect to the flow of the contrast image, the repeatability of the exam is limited by that maximum safe dosage. Also, the coil must either be repositioned or switched manually such that the active elements are in the region of the imaged scan station. Consequently, the time required to complete a conventional peripheral MRA study is of the order of 1.5 to 2.5 hours.
It would therefore be desirable to have a method and apparatus that is capable of optimally imaging the peripheral vasculature that includes computer control over table motion and coil selection and obtain images at locations where it is known that the contrast bolus is present.
The present invention provides a method and apparatus for optimal imaging of the peripheral vasculature that includes computer control over patient table motion and coil selection such that the signal-to-noise ratio (S/N) can be optimized at each of a number of scan locations that solves the aforementioned problems. The coordination with table motion allows accurate reproducibility of the different scan locations, thereby permitting optimal subtraction of a pre-contrast image mask from images obtained after the contrast bolus has been introduced. The method described is designed to pursue the passage of a bolus injection from the aorta down to the arteries in the lower extremities to acquire images of the arterial phase. Subsequent image acquisitions and mask subtractions can also allow post-processing of the data to generate venous phase images.
In accordance with one aspect of the invention, a method of MR imaging peripheral vasculature of a patient includes defining a given number of scan stations, with each of the scan stations positioned along the patient""s peripheral vasculature, and initially injecting a relatively small amount of contrast agent into the patient to initiate the passage of a test bolus through the patient""s peripheral vasculature. The passage of the test bolus is then tracked through the patient""s vasculature from one scan station to the next and the patient is moved fore and aft in the MR imaging apparatus to position the patient such that a desired scan station is within a field-of-view (FOV) of the MR imaging device based on the passage of the test bolus. The method also includes determining a travel time that it takes the test bolus to travel through each of the given number of scan stations, and thereafter, injecting additional contrast agent to form and pass an exam bolus through the patient""s peripheral vasculature. By using the test bolus travel time for each scan station, the passage of the exam bolus can be pursued through the patient""s peripheral vasculature and data can be acquired in each scan station during a period of time while the exam bolus is present.
In a preferred embodiment of the invention, the test bolus travel time to each station is initially determined. During the regular image acquisition phase of the MR exam, a pre-defined central k-space data block is acquired at each station in a time determined by the test bolus travel time to each station. If the test bolus travel time exceeds that for complete data at each station, the additional time is utilized by either acquiring additional higher k-space spatial frequency encoded data to improve spatial resolution, or to re-acquire the central k-space low spatial frequency data in order to improve image signal-to-noise ratio (SNR). Missing k-space data in each station, if any, would then be acquired at the end of the scan.
In accordance with another aspect of the invention, an MR system is disclosed that is capable of optimizing imaging of the patient peripheral vasculature and includes an MRI apparatus having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system, and an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly to thus acquire MR images. The MRI system of the present invention also includes a computer programmed to ensure placement of a movable patient table within the MRI apparatus and in a first scan station of a pre-defined given number of scan stations, and upon an indication that a test bolus has entered a given scan station of a patient, the computer is programmed to track the test bolus through that given scan station and record a travel time of the test bolus through that given scan station, then initiate patient table movement to a subsequent scan station. These steps are repeated for each subsequent scan station, and once complete, the computer returns the patient table to the first scan station. Upon an indication that an exam bolus has entered the patient, the computer activates the MRI apparatus to acquire at least central k-space MRI data of the patient within each of the scan stations for a period of time substantially equal to the test bolus travel time for that particular scan station, as previously recorded using the test bolus.
Accordingly, the method and apparatus of the present invention is used to control table position and move the patient from one scan station to another, and control coil element selection and set receiver and body coil transmitter gain parameters to optimize image S/N for each scan station. Additionally, the computer can adjust the acquisition matrix size or image field-of-view (FOV) at each station to optimize the image resolution on a per station basis.
Another feature of the preferred embodiment, is that once the bolus is introduced into the patient, the scan can be triggered using automatic bolus detection and triggering to assist in setting up the scan for the first scan station. After data acquisition of the first scan station, the computer can initiate movement of the patient table to the next station and select the appropriate receivers and adjust the transmitter and receiver gain settings appropriate for that particular scan station. This procedure is then repeated for each of the pre-programmed scan stations. In addition, by using a test bolus to determine the maximum imaging time available at each station, the MR data acquired is optimized to effectively visualize the arterial phase. The time available is used to acquire as many k-space lines as possible in each station before having to move to a subsequent station, with the central (low spatial frequency) k-space encoding lines acquired initially. It is noted that once sufficient k-space lines are acquired, or data acquisition at a particular station is complete, the system is capable of returning to a previous station to acquire additional k-space lines if time allows, or moving to a next station to acquire MR data, using the necessary table motion parameters and activation of the appropriate RF coil elements.
It is further noted that in peripheral run-off MRA, it is desired to image the peripheral arterial vasculature from the aorta at or above the level of the renal arteries, down to the lower extremities, including the feet. The present invention, as herein described, can also be used for a comprehensive assessment of the aorta, including the thoracic aorta, the abdominal aorta, and the aortoilliac segment. Where conventional MR imaging systems typically provide a maximum of 40-48 cm. image FOV, the present invention provides an effective imaging region of approximately 100-150 cm. FOV.